Hello and welcome to my AP Yi guide.
After playing (way too much, according to my friends) of this amazing one-ability wonder, and seeing alot of people not understanding the harmony of him at all, I decided to teach it to you, my way.
This is my first guide, and thus I am not quite familiar with the way of making fancy guides yet, and I believe if I was to make a huge variation of videos from things that can be easily understood by just simply telling them, I see no reason to add them..
If you are to rate my guide, please, rate it based on how the things are explained, not by how fancy it looks. Thank you.

The runes are flat AP (Outcounting the marks being MPen) and believe me, work perfectly throughout the game.
Now, ofcourse some 1.7k "Iwatchstreamstheysaynonotoflats" person will come and give me a mathematic lesson about the runes. Just no. Yi's early game is one of the strongest in game, and I like to keep it that way.

And, this way, a little mistake early can be corrected with a lvl 2 full hp meditate. With scaling AP runes this is not possible.

As explained in the notes, 21/0/9 masteries work perfectly for Yi, him being an almost untouchable little lane bully.

When it comes to blue buff, why would you want to take the xp and jungling speed from the jungler when you don't need it in the least bit.
If you have an energy jungler such as Lee Sin, Zed, Akali, I wouldn't even take it from them.
However, if I have something like Dr. Mundo, who needs it even less than I do, I usually tell my support/ad/top to take it, thus your team gets the most benefit.

If you're unfamiliar with yi, you could potentially pick up the blue, for the sakes of mistaking the timing off R+Q+Q+Q or if you are slow enough to react, and have to toggle ultimate again, after it runs out.

The 2 builds are pretty self-explanitory, I almost NEVER change the sequence of my item purchases, as Yi works the same way, in every situation.

One of the most common mistakes I see on AP Master Yi, is the stacking of Doran's Rings.
In the laning phase, you are able to harass with Alpha Strike and farm in one motion, making it possible to never run out of mana. Atleast if you know how to lasthit. And that is with one ring only. I wouldn't waste 1/2 of my Blasting Wand to take hp and manargn I don't even need.

Other than that, I leave up to your own judgment.

When it comes to the situational items, they should be picked/swapped in the lategame, where Yi's shine is starting to fade.

If the game lasts for over 60 minutes, and you have ALOT of money (Don't ever waste your money on elixirs with yi), Turn yourself into an AD Yi, as you will be more useful then with the incredible attack speed and damage. This is very situational, but I have won a few games by making this decision.

Alpha Strike Max this bad boy first, as it is your main source of damage, and the essence of AP Yi.

Meditate should be maxed second, for it grants amazing sustain and survival of clutch situations such as turret dives, both aggressive and defensive.

Wuju Style should never be leveled until you must, unless you get alot of early kills and are able to buy a sheen at level 4.

Highlander should be leveled as soon as possible, for it grants extra 2 seconds of Usain Bolt mode every time you level it.

Flash and Ignite are spells you do not want to change, for you are Yi, the early bully.

Highlander+ Flash+ Alpha Strike+[[ignite] is a combo too valuable for Yi to lose, as that includes 2 gapclosers, a huge nuke, and a DoT finisher, all of which can be executed in under a second.

HOWEVER, if you feel like the opponent is going to be very careful, I would recommend picking up Ghost for the fast roaming capability.

Pros:

One of the best early harassers along with huge sustain.
Can turn teamfights around, or clean up being in a 1v5 situation.
Hard to focus, due to him being untargetable when he Q's.
HUUUGE snowballer.
Usain Bolt.

Cons:

1 ability wonder, and you must land it correctly, otherwise you are utterly useless.
Needs ALOT of gold.
Meditate easily interrupted.
No CC.

Playing Ap Yi is one of the most hated and loved champions in Solo Queue.
Requiring a kill to be able to do multiple Qs, he is said to be the biggest killstealer in the game, which I totally agree with. EVEN if he cleans the enemy team alone, he isn't as loved as he should be. Damn Solo queue.

AP Yi is also good of a pick into the elohell scenario.
If you're the firstpick, and you feel like picking middle Yi, and someone trollpicks, Master Yi works every lane (except botlane) and jungle.

So, no need to stress about trollpicks and communicationless teams -> Yi goes where he pleases.

Farming is also one thing many (even experienced) AP yi's do wrong.
Because it is so different from farming with other champions, people tend to use Q way too much, or way too little.

The simplest way to use the Alpha Strike is this:

When the creep wave first comes, use basic attacks to lasthit the minions.
Then, when there's only 3 left, Alpha Strike the minions to get the harass onto the enemy AP carry.
If you use the it onto a minionwave that has MORE than 3 minions on it, the chances of damaging your opponent decrease radically.
When using it onto a minionwave of over 3 minions, it's all about luck whether you hit or not.

tl;dr:

Use Alpha Strike only when you are certain that you'll be able to hit the enemy AP carry, otherwise use basic attacks.